1. Field of the Invention
The disclosure is directed to a method of performing a handover procedure and a method of making a handover decision for device-to-device (D2D) communications and a control node thereof.
2. Description of Related Art
Considering device mobility in D2D communications, it is unable to support D2D handover in legacy Long Term Evolution (LTE) system. Although various methods have been proposed heretofore, there are several drawbacks, such as latency, extra resources, and extra signalling exchange in LTE system.
For example, when the Proximity-based Services (ProSe) capable user equipments (UEs) are performing ongoing D2D communication in the same cell and moving in the same direction, one of the ProSe capable UEs may be handed over to its neighboring cell. Therefore, the D2D communication may be interrupted as one of the ProSe capable UE performs LTE handover, and the ProSe capable UEs need to re-establish the D2D communication. When the other ProSe capable UE is handed over to its neighboring cell, the D2D communication may be interrupted again. When they both enter the same cell, the D2D communication needs to be established again. Therefore, how to provide more reliable D2D communications while maintaining continuity of the ProSe service is an important issue for people in the art.
D2D communication in LTE is a promising technology to realize Public Safety (PS) and commercial usage. Based on the scope of “Enhanced LTE Device to Device Proximity Services” in RP-150441, the continuity of ProSe service is considered as an important feature for enhancing D2D communications. Device mobility is one of the factor should be considered, which can affect the performance of continuity of ProSe service. When the ProSe capable UE in D2D communications moves across the cell boundary, the seamless handover is expected to provide the continuity for D2D communications service. However, a conventional LTE handover procedure is unable to support the continual ProSe service of D2D communications. For the purpose of handover with signalling minimization in the in-coverage scenario, how to provide the continuity for the ProSe service is challenging.
In addition, when ProSe capable UEs camping on different cells intend to perform D2D communications, the cross-cell D2D connection happens because they are geographically in proximity. There are two cases causing the situation of cross-cell D2D connection. FIG. 1a-1b are schematic diagrams illustrating the different cases of a conventional cross-cell D2D connection. In the first case, the ProSe capable UEs (i.e., UEs 110 and 120 as shown in FIG. 1a) originally camping on different cells (i.e., evolved node Bs (eNBs) 130 and 140 as shown in FIG. 1a). A cross-cell D2D communications should be enabled so that these ProSe capable UEs can directly communicate with each other, as shown in FIG. 1a. In the second case, as illustrated in FIG. 1b, the ProSe capable UEs (i.e., the UEs 110 and 120 as shown in FIG. 1b) perform ongoing D2D communications in the same cell at first. Due to mobility, one of the ProSe capable UE (i.e., the UE 120) may be handed over to its neighboring cell. Therefore, the D2D communications link may be interrupted, and then the moving ProSe capable UE (i.e., the UE 120) is handed over to the neighboring cell (i.e., the eNB 140). D2D communications is again established. That is, the second case can be viewed as the cross-cell D2D communications after a traditional handover procedure. However, the current solution to the second case may not be efficient since there is extra control signalling to provide the continuity for D2D communication service.
Considering the basic Rel-12 handover scenario in the radio access network, LTE handover can be divided into five phases. The five phases include measurement, handover decision, handover preparation, handover execution and handover completion. The basic Rel-12 handover scenario in the radio access network will be described with reference to Table 1, which indicates the handover related procedures.
TABLE 1Direction or RelatedProcedureEntitiesDescriptionMeasurementeNB → UESpecifies measurements toConfigurationbe performed by the UEMeasurement ReportUE → eNBindicates measurementresultsHandover Decisionsource eNBMakes decisions on targeteNBs and handover types(X2 or S1 handover)Handover PreparationVaries depending onPrepares forwarding pathHandover Executiona handover typeForwards dataHandover CompletionSwitches data path
Referring to Table 1, firstly, multiple eNBs specify measurements to be performed by the UE in Measurement Configuration procedure. Then, the UE measures the signal strength to its serving eNB and neighbor eNBs, and reports the results to the serving eNB, periodically or depending on a triggered measurement event, in Measurement Report procedure. Reporting criteria for Evolved Universal Terrestrial Radio Access (E-UTRA) report include Events A1, A2, A3, A4 and A5. The descriptions of the events used to trigger the LTE handover are as follows, where Event A3 is commonly used in triggering handovers. The descriptions of the events used to trigger the LTE hand over are provided as follows:                A1: The received signal strength from the serving cell becomes better than the threshold        A2: The received signal strength from the serving cell becomes worse than the threshold        A3: The received signal strength from the neighboring cell becomes offset better than the serving cell        A4: The received signal strength from the neighboring cell becomes better than the threshold        A5: The received signal strength from the neighboring cell becomes better than the threshold, and the received signal strength from the serving cell becomes worse than the threshold.        
As a measurement event is triggered, the UE measures the signal strength to the neighboring cells, and sends a Measurement Report message to its serving eNB (i.e., source eNB).
Next, the handover decision is made by the source eNB based on the measurement report from the UEs in Handover Decision procedure. When the event (i.e., one of the Events A1, A2, A3, A4 and A5) is reported, the source eNB decides what kind of handover to perform so as to switch to the target eNB, and then initiates a handover procedure. It is noted that the complete handover process may be categorized into many different types, and the type of X2 handover would be discussed herein. The X2 interface connects two eNBs (e.g., the source eNB and the target eNB). If there is an X2 connection between the source eNB and the target eNB, the X2 connection is available for handover, such that the X2 handover would be initiated. Once the handover is completed, the source eNB and the target eNB communicate with each other to control the handover without the interventions of the mobility management entity (MME).
Handover Preparation phase starts when the source eNB issues the handover request message to the target eNB. During this phase, the source eNB and the target eNB prepare for a handover. The source eNB sends the user's UE context (i.e., security context, quality of service (QoS) context, etc.) to the target eNB to check whether the target eNB is capable of providing the satisfactory service quality. If yes, the target eNB establishes a transport bearer (e.g., downlink (DL) packet forwarding bearer) for packet forwarding. Then, the target eNB allocates cell radio network temporary identifier (C-RNTI) value used by the UE to access the target eNB, and forwards the same information to the source eNB via X2 interface. Thus, the preparation phase is completed. At this time, the DL packet forwarding bearer forms a direct tunnel connecting the source eNB and the target eNB in X2 handover.
After that, the source eNB instructs the UE to perform a handover to the target eNB by sending a handover command message which includes all the information used to access the target eNB in Handover Execution procedure. In addition, the source eNB informs the target eNB which UL/DL packet should be received or sent by sending a sequence number (SN) status transfer message to the target eNB. The source eNB forwards the DL packets received from the Serving Gateway (S-GW) to the target eNB through the X2 transport bearer established between the source eNB and the target eNB. So the UE detaches from the source eNB and accesses to the target eNB. The target eNB becomes capable of sending and receiving packets once the UE has successfully accessed.
During the handover completion phase, the target eNB sends a path switch request message to the MME to inform that the UE has changed to attach to the target eNB. Once the UE completes its radio access to the target eNB successfully, the UE's bearer path (DL S1 bearer) is now connected to the target eNB rather then the source eNB. The MME informs the target eNB that the DL S1 bearer path has been modified. The target eNB sends a UE context release message to the source eNB, wherein the UE context release message includes the information of allowing the source eNB to release the resource.
Although the modification of the D2D communications handover must be done in respect to LTE handover, while it also needs to be backward compatible. In addition, since the D2D communications handover method for the problem of signalling minimization is not yet decided, how to provide more reliable D2D communications while maintaining continuity for the ProSe service is also an important issue. Though 3GPP has not yet specified this issue, it can be significant to support future D2D communications.